The Scala ArrayBuffer Class

If you’re an OOP developer coming to Scala from Java, the ArrayBuffer class will probably be most comfortable for you, so I’ll demonstrate it first. It’s a mutable sequence, so you can use its methods to modify its contents, and those methods are similar to methods on Java sequences.

To use an ArrayBuffer you must first import it:

import scala.collection.mutable.ArrayBuffer

After it’s imported into the local scope, you create an empty ArrayBuffer like this:

val ints = ArrayBuffer[Int]()
val names = ArrayBuffer[String]()

Once you have an ArrayBuffer you add elements to it in a variety of ways:

val ints = ArrayBuffer[Int]()
ints += 1
ints += 2

The REPL shows how += works:

scala> ints += 1
res0: ints.type = ArrayBuffer(1)

scala> ints += 2
res1: ints.type = ArrayBuffer(1, 2)

That’s just one way create an ArrayBuffer and add elements to it. You can also create an ArrayBuffer with initial elements like this:

val nums = ArrayBuffer(1, 2, 3)

Here are a few ways you can add more elements to this ArrayBuffer:

// add one element
nums += 4

// add two or more elements
nums += (5, 6)

// add elements from another collection
nums ++= List(7, 8)

You remove elements from an ArrayBuffer with the -= and --= methods:

// remove one element
nums -= 9

// remove two or more elements
nums -= (7, 8)

nums --= Array(5, 6)

Here’s what all of those examples look like in the REPL:

scala> nums += 4
res2: nums.type = ArrayBuffer(1, 2, 3, 4)

scala> nums += (5, 6)
res3: nums.type = ArrayBuffer(1, 2, 3, 4, 5, 6)

scala> nums ++= List(7, 8)
res4: nums.type = ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8)

scala> nums -= 9
res5: nums.type = ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8)

scala> nums -= (7, 8)
res6: nums.type = ArrayBuffer(1, 2, 3, 4, 5, 6)

scala> nums --= Array(5, 6)
res7: nums.type = ArrayBuffer(1, 2, 3, 4)

More ways to work with ArrayBuffer

There are many more ways to work with an ArrayBuffer:

val a = ArrayBuffer(1, 2, 3)         // ArrayBuffer(1, 2, 3)
a.append(4)                          // ArrayBuffer(1, 2, 3, 4)
a.append(5, 6)                       // ArrayBuffer(1, 2, 3, 4, 5, 6)
a.appendAll(Seq(7,8))                // ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8)
a.clear                              // ArrayBuffer()
val a = ArrayBuffer(9, 10)           // ArrayBuffer(9, 10)
a.insert(0, 8)                       // ArrayBuffer(8, 9, 10)
a.insert(0, 6, 7)                    // ArrayBuffer(6, 7, 8, 9, 10)
a.insertAll(0, Vector(4, 5))         // ArrayBuffer(4, 5, 6, 7, 8, 9, 10)
a.prepend(3)                         // ArrayBuffer(3, 4, 5, 6, 7, 8, 9, 10)
a.prepend(1, 2)                      // ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
a.prependAll(Array(0))               // ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
val a = ArrayBuffer.range('a', 'h')  // ArrayBuffer(a, b, c, d, e, f, g)
a.remove(0)                          // ArrayBuffer(b, c, d, e, f, g)
a.remove(2, 3)                       // ArrayBuffer(b, c, g)
val a = ArrayBuffer.range('a', 'h')  // ArrayBuffer(a, b, c, d, e, f, g)
a.trimStart(2)                       // ArrayBuffer(c, d, e, f, g)
a.trimEnd(2)                         // ArrayBuffer(c, d, e)

Please see the Scala Cookbook for more details on the ArrayBuffer class.